1. Field of the Invention
This invention generally relates to optical and electro-optical packing and more particularly to a package for high frequency components.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee of ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals that are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and dispersion compensation by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are separated into channels detected by photodetectors, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for short and intermediate range optical links to allow high speed data transfers within various components of a central office. Currently such links require the same expensive circuits and lasers used in long-range optical links. What is needed is a short- and intermediate-range high data rate optical communication system that does not require the cost and complexity of prior art devices and which preferably can be achieved with a reduced form factor compared with prior art devices.
The present invention provides a method and apparatus for packaging high frequency electrical and/or electro-optical components. The present invention provides a package which may be surface mounted on a board with other electrical components. The shielding provided by the package minimizes electromagnetic interference with other electrical components on the board. The package includes a controlled impedance I/O interface for coupling with the electrical and/or electro-optical component(s) in the package. The package interface may also include a differential I/O capability to further control electromagnetic fields generated at the interface. Additionally, the package may include an optical link provided by one or more optical fibers extending from the package. These features allow the package to be used for a range of high-frequency components including optical transmitters and receivers. In combination these features result in a package that may be fabricated with relatively low cost and a reduced form factor when compared with prior art packages.
In an embodiment of the invention the package for high frequency components, includes a cover, a multilayer base and an electrical component. The cover includes a lower surface. The multilayer base includes a dielectric layer between a top layer and a bottom layer. The top layer and the bottom layer each include an electrically conductive ground plane portion, I/O pad portion, and differential I/O pad portion. Each corresponding portion is coupled to one another by vias within the dielectric layer. The multilayer base couples to the lower surface of the cover and the combined multilayer base and cover define an electrically shielded cavity within the package. The electrical component is located within the electrically shielded cavity. The electrical component is differentially coupled electrically though the differential I/O pad portion on the top layer to a corresponding differential pad portion on the bottom layer. RF energy radiated from the electrical component during operation is substantially contained within the package by the electrically shielded cavity.
In an alternate embodiment of the invention a method for fabricating a package for retaining an electrical component with a plurality of connection points is disclosed. The method for fabricating includes the acts of:
providing a dielectric substrate with a top surface and a bottom surface;
forming a conductive ground plane, pad, and differential pad portions on the top surface and the bottom surface of the dielectric substrate;
electrically coupling corresponding ones of the ground plane, pad, and differential pad portions on the top surface and the bottom surface of the dielectric substrate to provide controlled impedance external connections to the electrical component within the package;
bonding an electrically conductive cover to form an electrically shielded cavity around the electrical component.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.